1. Field of the Invention
The invention relates to a new composite system in which the interface/interphase between the reinforcing material and the matrix is remendable. Following an interfacial failure heating the composite breaks and reforms Diels-Alder bonds across the interface in order to restore the properties of the composite system.
2. Brief Description of the Prior Art
Materials that can recover mechanical properties are highly desirable. Because remendable materials both repair and prevent the propagation of cracks on the micro scale, they offer the potential for increased durability, safety, and cost efficiency for many applications. Two approaches for mending or healing polymer networks have captured much attention. In one method, polymer networks are made to self-heal by incorporating microcapsules filled with uncured resin as a secondary phase. Upon fracture, the microcapsules rupture and release resin, which polymerizes to heal the fracture. Often, a catalyst is also included in the polymer network to initiate polymerization of the uncured resin when released. The other method relies on incorporation of reversible bonds into the polymer network. The reversible nature of these linkages allows for network remodeling at the damage site.
Polymers used in load-bearing applications are often reinforced with rigid particles or fibers to improve their specific strength. Adhesion between the reinforcement and the polymer matrix is essential for load transfer from the polymer matrix to the reinforcement material. However, the difference in properties between the matrix material and the reinforcement material makes the interface between the two a location for stress concentration and eventual crack or fracture formation. Fatigue, whether mechanically or thermally induced, leads to growth of such cracks and is the major cause of mechanical failure in composites. Surface treatments and application of a chemical sizing are conventional options for improving interfacial adhesion and durability of composites.
Despite this, fatigue will eventually result in interfacial failure even with the most durable surface treatment. However, if an interface can form reversible covalent bonds between the polymer network and the reinforcement material, this interface will be capable of healing, resulting in an improvement in the mechanical properties and an extension of the lifetime of the composite. Another approach for healing of interfacial failure via the introduction of healing agent-filled capsules is described in U.S. Patent application publication no. US 2010/0075134 A1.
The Diels-Alder reaction is a cycloaddition of a diene and a dienophile. In the case of furan and maleimide, furan acts as the diene and maleimide acts as the dienophile. The reaction favors the product (Diels-Alder adduct) below 60° C. and the reactants (furan and maleimide) above 90° C. Between 60 and 90° C. equilibria exist between various relative concentrations of the products and reactants. FIG. 1 shows this reversible reaction. A representation of the concept of this invention is shown in FIG. 2.
Thus, there is a need for composite materials which can repair themselves. These and other objects of the present invention will be apparent from the summary and detailed description which follow.